The ground-state Na+ affinities of a series of substituted acetophenones: a DFT study

A detailed study of Na⁺ affinities of a series of para-substituted acetophenones and their O–Na⁺ counterparts was performed using density functional theory [Becke, Lee, Yang and Parr (B3LYP)] method using 6-311G(d,p) basis sets with complete geometry optimisation. The gas-phase O–Na⁺ complex formation turns out to be an exothermic case and the local stereochemical disposition of Na⁺ is found to be almost the same in each case. The presence of the para-substituent is seen to cause very little change in the Na⁺ affinity relative to the unsubstituted acetophenones. Electron-releasing p-substituents increase it by 0.0105 hartree and electron-withdrawing p-substituents decrease it by 0.011 hartree. Computed Na⁺ affinities are sought to be correlated with a number of computed system parameters such as the net charge on the Na⁺ and the carbonyl oxygen of the Na⁺ complexes and the net charge on the carbonyl oxygen of the free bases. The energetics, structural and electronic properties of the complexes indicate that the interaction between the Na⁺ ion and a carbonyl base is predominantly an ion–dipole attraction and the ion-induced dipole interaction as well rather than a covalent interaction.     

Selectivity and sensitivity to hydrogen ion blocking of batrachotoxin-modified sodium channels in nerve fiber membrane

Currents through batrachotoxin-modified sodium channels were measured by the voltage clamp method on the Ranvier node membrane. In experiments with replacement of Na⁺ in the external solution by K⁺ or NH ₄ ⁺ the following series of permeabilities, determined as reversal potentials according to the equation of a static field, was obtained — P_Na: \(P_{NH_4 }\) :P_K=1:0.47:0.19. The relative permeability for H⁺ was determined by measuring currents after replacement of Na⁺ in the external solution by nonpenetrating choline ions and lowering pH to 3.7–3.8. The ratio p_H/p_Na for sodium channels modified by batrachotoxin averaged 528±46. Modified channels were less sensitive to the blocking action of H⁺ than normal sodium channels. The difference in the effective values of pK of the acid group of normal and modified channels was 0.40–0.45.     

Rapid effects of nerve growth factor on the Na,K-pump in rat pheochromocytoma cells

Nerve growth factor (NGF) induces neuronal differentiation of rat pheochromocytoma cells (PC12). Here we show that NGF causes a stimulation of Na⁺,K⁺-pump mediated K⁺ influx, with a maximum at 30 min after addition of NGF. The stimulation of the Na⁺,K⁺-pump is completely blocked by the Na⁺-flux inhibitor amiloride (0.2 mM) and can be mimicked by the Na⁺ ionophore monensin. These results suggest that NGF causes a rapid enhancement of Na⁺ influx leading to an activation of the Na⁺,K⁺-pump, a mechanism similar to the action of other growth factors.     

Kinetic properties of sodium transport pathways in the river lamprey <Emphasis Type="Italic">Lampetra fluviatilis</Emphasis> erythrocytes

To activate Na⁺/H⁺ exchange, intracellular pH (pH_i) of erythrocytes of the river lamprey Lampetra fluviatilis were changed from 6 and 8 using nigericin. The Na⁺/H⁺ exchanger activity was estimated from the values of amiloride-sensitive components of Na⁺ (²²Na) inflow or of H⁺ outflow from erythrocytes. Kinetic parameters of the carrier functioning were determined by using Hill equation. Dependence of Na⁺ and H⁺ transport on pH_i value is described by hyperbolic function with the Hill coefficient value (n) close to 1. Maximal rate of ion transport was within the limits of 9–10 mmol/l cells/min, and the H⁺ concentration producing the exchanger 50% activation amounted to 0.6–1.0 μM. Stimulation of H⁺ outcome from acidified erythrocytes (pH_i 5.9) with increase of H⁺ concentration in the incubation medium is described by Hill equation with n value of 1.6. Concentration Na⁺ for the semimaximal stimulation of H⁺ outcome amounted to 10 mM. The obtained results indicate the presence in lamprey erythrocytes of only binding site for H⁺ from the cytoplasm side and the presence of positive cooperativity in Na⁺-binding from the extracellular side of the Na⁺/H⁺ exchanger. Na⁺ efflux from cells in the Na⁺-free medium did not change at a 10-fold increase of H⁺ concentration in the incubation medium. The presented data indicate differences of kinetic properties of the lamprey erythrocyte Na⁺/H⁺ exchanger and of this carrier isoforms in mammalian cells. In intact erythrocytes the dependence of the amiloride-sensitive Na⁺ inflow on its concentration in the medium is described by Hill equitation with n 1.6. The Na⁺ concentration producing the 50% transport activation amounted to 39 mM and was essentially higher as compared with that in acidified erythrocytes. These data confirm conception of the presence of two amiloride-sensitive pathways of Na+ transport in lamprey erythrocytes.     

Short-latency ionic effects of nerve growth factor deprivation and readministration on ganglionic cells

Nerve growth factor (NGF) is likely to exert its trophic action on dorsal root ganglion (DRG) and on sympathetic ganglion neurons by controlling a crucial function of these cells. This function would in turn regulate other cellular machineries and, ultimately, lead to the traditional NGF consequences, such as survival and neuritic growth. A corollary of this view is that the key to NGF action must lie in short-latency events, occurring within minutes of NGF administration. Chick embryo DRG dissociates have proved to be an effective experimental system to investigate short-latency responses to NGF, in that (1) measurable functional deficits develop over 6 h of NGF deprivation in vitro and (2) delayed presentation of NGF promptly and fully restores the defective function. The first deficit observed in this experimental system, a decline in RNA-labeling capability, led to the recognition that NGF controls the transport of selected exogenous substrates, all of which are Na⁺-coupled and depend on an Na⁺ gradient across the neuronal membrane. Subsequent work showed that NGF controlled such transport systems by actually regulating the neuronal ability to control intracellular Na⁺. Under NGF deprivation, the DRG cells accumulate Na⁺ to levels that reflect, and presumably equate, the extracellular Na⁺ concentrations. Conversely, on delayed NGF administration, the accumulated Na⁺ is actively extruded to an extent and at a speed that depends on the NGF concentration. The Na⁺ response is elicited by both Beta and 7S NGF, but not by other proteins tested. All ganglionic systems that display a requirement for exogenous NGF in culture have also displayed the Na⁺ response to NGF. The Na⁺ response is grossly paralleled by a K⁺ response. DRG dissociates, in which intracellular K⁺ has been pre-equilibrated with extracellular ⁸⁶Rb⁺, lose their ⁸⁶Rb⁺ over 6 h of NGF deprivation and restore it on delayed NGF administration. The regulation by NGF of mechanisms controlling intracellular Na⁺ and K⁺ levels in their target neurons is likely to occupy an early and fundamentl place in the sequence of events underlying the mode of action of this factor.     

Quantitative Characterization of Non-Classic Polarization of Cations on Clay Aggregate Stability

Soil particle interactions are strongly influenced by the concentration, valence and ion species and the pH of the bulk solution, which will also affect aggregate stability and particle transport. In this study, we investigated clay aggregate stability in the presence of different alkali ions (Li⁺, Na⁺, K⁺, and Cs⁺) at concentrations from10⁻⁵ to 10⁻¹ mol L⁻¹. Strong specific ion effects on clay aggregate stability were observed, and showed the order Cs⁺>K⁺>Na⁺>Li⁺. We found that it was not the effects of ion size, hydration, and dispersion forces in the cation–surface interactions but strong non-classic polarization of adsorbed cations that resulted in these specific effects. In this study, the non-classic dipole moments of each cation species resulting from the non-classic polarization were estimated. By comparing non-classic dipole moments with classic values, the observed dipole moments of adsorbed cations were up to 10⁴ times larger than the classic values for the same cation. The observed non-classic dipole moments sharply increased with decreasing electrolyte concentration. We conclude that strong non-classic polarization could significantly suppress the thickness of the diffuse layer, thereby weakening the electric field near the clay surface and resulting in improved clay aggregate stability. Even though we only demonstrated specific ion effects on aggregate stability with several alkali ions, our results indicate that these effects could be universally important in soil aggregate stability.     

Control of the Amiloride-Sensitive Na+ Current in Salivary Duct Cells by Extracellular Sodium

We have previously reported that intralobular salivary duct cells contain an amiloride-sensitive Na⁺ conductance (probably located in the apical membranes). Since the amiloride-sensitive Na⁺ conductances in other tight epithelia have been reported to be controlled by extracellular (luminal) Na⁺, we decided to use whole-cell patch clamp techniques to investigate whether the Na⁺ conductance in salivary duct cells is also regulated by extracellular Na⁺. Using Na⁺-free pipette solutions, we observed that the whole-cell Na⁺ conductance increased when the extracellular Na⁺ was increased, whereas the whole-cell Na⁺ permeability, as defined in the Goldman equation, decreased. The dependency of the whole-cell Na⁺ conductance on extracellular Na⁺ could be described by the Michaelis-Menten equation with a K _m of 47.3 mmol/1 and a maximum conductance (G _max) of 2.18 nS. To investigate whether this saturation of the Na⁺ conductance with increasing extracellular Na⁺ was due to a reduction in channel activity or to saturation of the single-channel current, we used fluctuation analysis of the noise generated during the onset of blockade of the Na⁺ current with 200 μmol/l 6-chloro-3,5-diaminopyrazine-2-carboxamide. Using this technique, we estimated the single channel conductance to be 4 pS when the channel was bathed symmetrically in 150 mmol/l Na⁺ solutions. We found that Na⁺ channel activity, defined as the open probability multiplied by the number of available channels, did not alter with increasing extracellular Na⁺. On the other hand, the single-channel current saturated with increasing extracellular Na⁺ and, consequently, whole-cell Na⁺ permeability declined. In other words, the decline in Na⁺ permeability in salivary duct cells with increasing extracellular Na⁺ concentration is due simply to saturation of the single-channel Na⁺ conductance rather than to inactivation of channel activity. Received: 27 July 1995/Revised: 7 December 1995     

Cation flux in the ehrlich ascites tumor cell evidence for Na-for-Na and K-for-K exchange diffusion

In a previous study, evidence was presented for an external Na⁺-dependent, ouabain-insensitive component of Na⁺ efflux and an external K⁺-dependent component of K⁺ efflux in the Ehrlich ascites tumor cell. Evidence is now presented that these components are inhibited by the diuretic furosemide and that under conditions of normal extracellular Na⁺ and K⁺ they represent Na⁺-for-Na⁺ and K-⁺for-K⁺ exchange mechanisms. Using ⁸⁶Rb to monitor K⁺ movements, furosemide is shown to inhibit an ouabain-insensitive component of Rb⁺ influx and a component of Rb⁺ efflux, both representing approx. 30% of the total fux. Inhibition of Rb⁺ efflux is greatly reduced by removal of extracellular K⁺. Furosemide does not alter steady-state levels of intracellular K⁺ and it does not prevent cells depleted of K⁺ by incubation in the cold from regaining K⁺ upon warming. Using ²²Na to monitor Na⁺ movements, furosemide is shown to inhibit an ouabain-insensitive component of unidirectional Na⁺ efflux which represents approx. 22% of total Na⁺ efflux. Furosemide does not alter steady-state levels of intracellular Na⁺ and does not prevent removal of intracellular Na⁺ upon warming from cells loaded with Na⁺ by preincubation in the cold. The ability of furosemide to affect unidirectional Na⁺ and K⁺ fluxes but not net fluxes is consistent with the conclusion that these components of cation movement across the cell membrane represent one-for-one exchange mechanisms. Data are also presented which demonstrate that the uptake of α-aminoisobutyrate is not affected by furosemide. This indicates that these components of cation flux are not directly involved in the Na⁺-dependent amino acid transport system A.     

Na---Ca exchange in squid optic nerve membrane vesicles is activated by internal calcium

The role of intracellular Ca²⁺ as essential activator of the Na⁺---Ca²⁺ exchange carrier was explored in membrane vesicles containing 67% right-side-out and 10% inside-out vesicles, isolated from squid optic nerves. Vesicles containing 100 μM free calcium exhibited a 2-fold increase in the initial rate of Na_i⁺-dependent Ca²⁺ uptake as compared with vesicles where intravesicular calcium was chelated by 2 mM EGTA or 10 mM HEDTA. The activatory effect exerted by intravesicular Ca²⁺ on the reverse mode of Na⁺---Ca²⁺ exchange (i.e. Na_i⁺---Ca₀²⁺ exchange) is saturated at about 100 μM Ca_i²⁺ and displays an apparent K_1/2 of 12 μM. Intravesicular Ca²⁺ produced activation of Na_i⁺---Ca₀²⁺ exchange activity rather than an increase in Ca²⁺ uptake due to Ca²⁺---Ca²⁺ exchange. The presence of Ca_i²⁺ was essential for the Na_i⁺-dependent Na⁺ influx, a partial reaction of the Na⁺---Ca²⁺ exchanger. In fact, the Na⁺ influx levels in vesicles loaded with 2 mM EGTA were close to those expected from diffusional leak while in vesicles containing Ca_i²⁺ an additional Na⁺---Na⁺ exchange was measured. The results suggest that in nerve membrane vesicles Ca²⁺ at the inner aspect of the membrane acts as an activator of the Na⁺---Ca²⁺ exchange system.     

Partial purification and characterization of (Na++K+)-ATPase from garfish olfactory nerve axon plasma membrane

Summary The (Na⁺+K⁺)-ATPase of garfish olfactory nerve axon plasma membrane was purified about sixfold by treatment of the membrane with sodium dodecyl sulfate followed by sucrose density gradient centrifugation. The estimated molecular weights of the two major polypeptide components of the enzyme preparation on sodium dodecyl sulfate gels were 110,000 and 42,000 daltons, which were different from those of the corresponding peptides of rabbit kidney (Na⁺+K⁺)-ATPase. No carbohydrate was detected in the 42,000-dalton component either by the periodic acid-Schiff reagent or by the more sensitive concanavalin A-peroxidase staining procedure. The molecular properties of the garfish (Na⁺+K⁺)-ATPase, such as theK _m for ATP, pH optimum, energies of activation, Na and K ion dependence and vanadium inhibition, were, however, similar to those of the kidney enzyme.The partially purified garfish (Na⁺+K⁺)-ATPase was reconstituted into phospholipid vesicles by a freeze-thaw-sonication procedure. The reconstituted enzyme was found to catalyze a time and ATP dependent²²Na⁺ transport. The ratio of²²Na⁺ pumped to ATP hydrolyzed was about 1; under the same reconstitution and assay conditions, eel electroplax (Na⁺+K⁺)-ATPase, however, gave a²²Na⁺ pumped to ATP hydrolyzed ratio of nearly 3.     

Beyond non-integer Hill coefficients: A novel approach to analyzing binding data,applied to Na+-driven transporters

Prokaryotic and eukaryotic Na⁺-driven transporters couple the movement of one or more Na⁺ ions down their electrochemical gradient to the active transport of a variety of solutes. When more than one Na⁺ is involved, Na⁺-binding data are usually analyzed using the Hill equation with a non-integer exponent n. The results of this analysis are an overall K_d-like constant equal to the concentration of ligand that produces half saturation and n, a measure of cooperativity. This information is usually insufficient to provide the basis for mechanistic models. In the case of transport using two Na⁺ ions, an n < 2 indicates that molecules with only one of the two sites occupied are present at low saturation. Here, we propose a new way of analyzing Na⁺-binding data for the case of two Na⁺ ions that, by taking into account binding to individual sites, provides far more information than can be obtained by using the Hill equation with a non-integer coefficient: it yields pairs of possible values for the Na⁺ affinities of the individual sites that can only vary within narrowly bounded ranges. To illustrate the advantages of the method, we present experimental scintillation proximity assay (SPA) data on binding of Na⁺ to the Na⁺/I⁻ symporter (NIS). SPA is a method widely used to study the binding of Na⁺ to Na⁺-driven transporters. NIS is the key plasma membrane protein that mediates active I⁻ transport in the thyroid gland, the first step in the biosynthesis of the thyroid hormones, of which iodine is an essential constituent. NIS activity is electrogenic, with a 2:1 Na⁺/I⁻ transport stoichiometry. The formalism proposed here is general and can be used to analyze data on other proteins with two binding sites for the same substrate.     

Identification of Electric-Field-Dependent Steps in the Na+,K+-Pump Cycle

The charge-transporting activity of the Na⁺,K⁺-ATPase depends on its surrounding electric field. To isolate which steps of the enzyme’s reaction cycle involve charge movement, we have investigated the response of the voltage-sensitive fluorescent probe RH421 to interaction of the protein with BTEA (benzyltriethylammonium), which binds from the extracellular medium to the Na⁺,K⁺-ATPase’s transport sites in competition with Na⁺ and K⁺, but is not occluded within the protein. We find that only the occludable ions Na⁺, K⁺, Rb⁺, and Cs⁺ cause a drop in RH421 fluorescence. We conclude that RH421 detects intramembrane electric field strength changes arising from charge transport associated with conformational changes occluding the transported ions within the protein, not the electric fields of the bound ions themselves. This appears at first to conflict with electrophysiological studies suggesting extracellular Na⁺ or K⁺ binding in a high field access channel is a major electrogenic reaction of the Na⁺,K⁺-ATPase. All results can be explained consistently if ion occlusion involves local deformations in the lipid membrane surrounding the protein occurring simultaneously with conformational changes necessary for ion occlusion. The most likely origin of the RH421 fluorescence response is a change in membrane dipole potential caused by membrane deformation.     

Identification of Electric-Field-Dependent Steps in the Na,K-Pump Cycle

The charge-transporting activity of the Na⁺,K⁺-ATPase depends on its surrounding electric field. To isolate which steps of the enzyme’s reaction cycle involve charge movement, we have investigated the response of the voltage-sensitive fluorescent probe RH421 to interaction of the protein with BTEA (benzyltriethylammonium), which binds from the extracellular medium to the Na⁺,K⁺-ATPase’s transport sites in competition with Na⁺ and K⁺, but is not occluded within the protein. We find that only the occludable ions Na⁺, K⁺, Rb⁺, and Cs⁺ cause a drop in RH421 fluorescence. We conclude that RH421 detects intramembrane electric field strength changes arising from charge transport associated with conformational changes occluding the transported ions within the protein, not the electric fields of the bound ions themselves. This appears at first to conflict with electrophysiological studies suggesting extracellular Na⁺ or K⁺ binding in a high field access channel is a major electrogenic reaction of the Na⁺,K⁺-ATPase. All results can be explained consistently if ion occlusion involves local deformations in the lipid membrane surrounding the protein occurring simultaneously with conformational changes necessary for ion occlusion. The most likely origin of the RH421 fluorescence response is a change in membrane dipole potential caused by membrane deformation.     

Fluctuation analysis of Na+ channels modified by batrachotoxin in myelinated nerve

(1) Single myelinated nerve fibres of Rana esculenta were treated with the steroidal alkaloid batrachotoxin, and Na⁺ currents and Na⁺-current fluctuations were measured near the resting potential under voltage-clamp conditions. Between test pulses fibres were held at hyperpolarizing membrane potentials. (2) The spectral density of Na⁺-current fluctuations was fitted by the sum of a $\text{1}\text{f}$ component and a Lorentzian function. The time constant $\text{τ}{}_{\mathrm{<mtext>c</mtext>}}\text{= 1/(2π?}{}_{\mathrm{<mtext>c</mtext>}}\text{)}$ obtained from the corner frequency ?_c of the Lorentzian function approximately agreed with the activation time constant τ_m of the macroscopic currents. (3) The conductance γ of a single Na⁺ channel modified by batrachotoxin was calculated from the integral of the Lorentzian function and the steady-state Na⁺ current. At the resting potential V = O we obtained γ = 1.6 pS, higher γ-values of 3.2 and 3.45 pS were found at V = ?8 and ?16 mV, respectively. (4) The conductance of a modified Na⁺ channel is significantly lower than the values 6.4 to 8.85 pS reported in the literature for normal Na⁺ channels. Hence, our experiments are in agreement with the view that batrachotoxin acts in an ‘all-or-none’ manner on Na⁺ channels and creates a distinct population of modified channels.     

A non-equilibrium thermodynamic theory of leakage of complexed Na+ from muscle,with NMR evidence that the non-complexed fraction of muscle Na+ is intra-vacuolar rather than extra-cellular

Recent experimental evidence has provided increasing support for the hypotheses that 60 to 80 per cent of intracellular Na⁺ exists in a complexed state, and that intracellular water exists in a semi-organized, non-liquid state having low solubility for Na⁺. Using these postulates, a previous crude theory of Na⁺ leakage from the cell based on electronion conduction analogies has been redeveloped in a more complete and detailed fashion, following a non-equilibrium thermodynamic approach. The theory, which is based on the postulate of almost 100 per cent complexing of intracellular Na⁺, predicts that Na⁺ leakage from muscle should conform to the Elovich equation, which closely agrees with experiment, despite the fact that experiments indicate that 20 to 40 per cent of muscle Na⁺ isnot complexed. To resolve this apparent paradox, the leakage of complexed and non-complexed Na⁺ from muscle was measured by nuclear magnetic resonance (NMR). The non-complexed Na⁺ leaked much more slowly than the complexed Na⁺, suggesting that the non-complexed Na⁺ may be confined within vacuoles surrounded by an activation energy barrier at the vacuolar membrane. This implies that the measured curves of Na⁺ leakage showing Elovich kinetics are due mostly to leakage of complexed Na⁺ as the theory requires, and that the leakage of 20 to 40 per cent non-complexed Na⁺ is mostly delayed until later times.     

Molecular dynamics study of Na+ transportation in a cyclic peptide nanotube and its influences on water behaviors in the tube

The dynamics of Na⁺ transportation in a transmembrane cyclic peptide nanotube of 8?×?(WL)₄/POPE has been simulated. The curve of PMF (potential of mean force) for Na⁺ moving through the tube, based on ABF (adaptive biasing force) method, indicates that Na⁺ possesses lower free energy in an α-plane region than in a mid-plane one. It was found that Na⁺ would desorb one or two water molecules in the first solvation shell when entering the tube and later maintain in a solvation state. The average numbers of water molecules around Na⁺ are 4.50, 4.09 in the first solvation shell, and 3.10, 4.08 in the second one for Na⁺ locating in an α-plane zone and a mid-plane region, respectively. However, water molecules far away from Na⁺ location still nearly arrange in a form of 1-2-1-2 file. The dipole orientations of water molecules in the regions of gaps 1 and 7 display “D-defects”, resulted from the simultaneous electrostatic potentials generated by Na⁺ and the bare carbonyls at the tube mouths. Such “D-defects” accommodate the energetically favorable water orientations thereby.

长苞香蒲对人工盐碱湿地Na+和K+的吸收与转运特征

为揭示长苞香蒲(Typha domingensis)对盐生湿地生态系统中Na⁺和K⁺的吸收与转运特征,探讨长苞香蒲对盐生湿地的生态修复效果,该研究采用人工模拟盐生湿地的方法,设置CK(对照)、T1(浇灌100 mmol·L^-1盐水)、T2(浇灌200 mmol·L^-1盐水)及T3(浇灌300 mmol·L^-1盐水)4种不同盐浓度的人工湿地生态系统,并分别于5月5日(开始盐胁迫处理,S0)、5月30日(S1)、6月30日(S2)和7月30日(S3)测量其株高和干重、植株地上与地下部分Na⁺和K⁺的含量以及底泥和水体中Na⁺和K⁺的含量以分析长苞香蒲对盐碱湿地的脱盐作用。结果表明：(1)各处理的长苞香蒲的株高和干重随着处理时间的延长呈增加趋势,但与CK相比,各处理生长量随盐浓度升高出现下降趋势。(2)高浓度盐处理(T3)使长苞香蒲的地上部分和地下部分的Na⁺分别增加了2.5...     

Rescue of Na+ Affinity in Aspartate 928 Mutants of Na+,K+-ATPase by Secondary Mutation of Glutamate 314

The Na⁺,K⁺-ATPase binds Na⁺ at three transport sites denoted I, II, and III, of which site III is Na⁺-specific and suggested to be the first occupied in the cooperative binding process activating phosphorylation from ATP. Here we demonstrate that the asparagine substitution of the aspartate associated with site III found in patients with rapid-onset dystonia parkinsonism or alternating hemiplegia of childhood causes a dramatic reduction of Na⁺ affinity in the α1-, α2-, and α3-isoforms of Na⁺,K⁺-ATPase, whereas other substitutions of this aspartate are much less disruptive. This is likely due to interference by the amide function of the asparagine side chain with Na⁺-coordinating residues in site III. Remarkably, the Na⁺ affinity of site III aspartate to asparagine and alanine mutants is rescued by second-site mutation of a glutamate in the extracellular part of the fourth transmembrane helix, distant to site III. This gain-of-function mutation works without recovery of the lost cooperativity and selectivity of Na⁺ binding and does not affect the E₁-E₂ conformational equilibrium or the maximum phosphorylation rate. Hence, the rescue of Na⁺ affinity is likely intrinsic to the Na⁺ binding pocket, and the underlying mechanism could be a tightening of Na⁺ binding at Na⁺ site II, possibly via movement of transmembrane helix four. The second-site mutation also improves Na⁺,K⁺ pump function in intact cells. Rescue of Na⁺ affinity and Na⁺ and K⁺ transport by second-site mutation is unique in the history of Na⁺,K⁺-ATPase and points to new possibilities for treatment of neurological patients carrying Na⁺,K⁺-ATPase mutations.     

Na+ and K+ transport in excised soybean roots

Uptake, accumulation and xylem transport of K⁺ and Na⁺ in excised roots of soybean were investigated by use of a perfusion technique. This technique permitted independent quantification of, on the one hand, entry of ions into the roots and their transport through the cortex to the xylem vessels, and on the other hand reabsorption from the xylem vessels to the neighbouring cells and the external medium. Data are consistent with a low degree of selective uptake of K⁺ over Na⁺. However, Na⁺ depletion of the xylem stream by reabsorption limits, although weakly, its translocation to the shoots. Na⁺ reabsorbed is for a great part reexcreted into the external medium. The low efficiency of these processes is discussed in relation to the Na⁺ sensitivity of soybean.     

Computer reconstruction of the spread of excitation in nerve terminals with inhomogeneous channel distribution

A direct numerical integration method, as modified by Du Fort and Frankel (1953), has been used to solve the partial differential equation system which describes the spread of action potential in a mammalian nerve terminal. Branching of the terminal as well as inhomogeneous distributions of Na⁺ and K⁺ voltage-dependent channels (Brigant and Mallart 1982) have been incorporated in the model.Using the channel densities and the kinetic parameters measured in the node of Ranvier, the depolarization in the terminal branches has an amplitude of only 60% of the action potential in the node. Furthermore, the time courses of the calculated membrane currents differ considerably from the ones measured by Brigant and Mallart (1982) and by Konishi and Sears (1984).Increasing the Na⁺ and K⁺ channel densities may considerably increase the terminal depolarization and also reproduce qualitatively the current waveforms observed experimentally. The model can also reproduce some of the effects of pharmacological channel blocks.The simulation allows a new interpretation of the different components of membrane current along the terminal.